C# LINQ问题：当NumberOfSchools为0时如何正确设置Year字段？

需求是获取yearList中每一年对应的毕业生毕业院校的不同数量数据，示例如下：

nameList: new[]
{
    new Entrant {LastName = "Name", SchoolNumber = 12, Year = 2019},
    new Entrant {LastName = "Name", SchoolNumber = 13, Year = 2019},
    new Entrant {LastName = "Name", SchoolNumber = 12, Year = 2019},
    new Entrant {LastName = "Name", SchoolNumber = 14, Year = 2016},
    new Entrant {LastName = "Name", SchoolNumber = 15, Year = 2016},
    new Entrant {LastName = "Name", SchoolNumber = 12, Year = 2018},
    new Entrant {LastName = "Name", SchoolNumber = 12, Year = 2018},
    new Entrant {LastName = "Name", SchoolNumber = 13, Year = 2018},
    new Entrant {LastName = "Name", SchoolNumber = 12, Year = 2017},
    new Entrant {LastName = "Name", SchoolNumber = 12, Year = 2017}
},
yearList: new[] { 2020, 2017, 2018 },
expected: new[]
{
    new YearSchoolStat {NumberOfSchools = 0, Year = 2020},
    new YearSchoolStat {NumberOfSchools = 1, Year = 2017},
    new YearSchoolStat {NumberOfSchools = 2, Year = 2018}
});

我实现了部分逻辑，但当NumberOfSchools为0时，Year字段会被设为0，不符合预期。当前代码如下：

var groupedYear = yearList.GroupBy(x => x);
var groupedSchoolNumber = groupedYear.Select(y => nameList.Where(x => x.Year == y.Key)).Select(group => group.GroupBy(item => item.SchoolNumber));
var result = groupedSchoolNumber.Select(s => new YearSchoolStat { NumberOfSchools = s.Count(), Year = s.Select(y => y.Select(x => x.Year).FirstOrDefault()).FirstOrDefault() }).OrderBy(x => x.NumberOfSchools);
return result;

期望无数据（NumberOfSchools=0）时，Year能正确对应yearList中的年份。

原代码的问题在于，当某个年份没有对应数据时，你试图从空分组中提取Year，自然会得到默认值0。正确的做法是直接保留yearList中的年份值，不要从过滤后的结果中去获取Year。

修正后的代码：

var result = yearList.Select(year => new YearSchoolStat
{
    Year = year,
    NumberOfSchools = nameList
        .Where(x => x.Year == year)
        .Select(x => x.SchoolNumber)
        .Distinct()
        .Count()
})
.OrderBy(x => x.NumberOfSchools);

return result;

代码说明 #

• 直接遍历yearList中的每一个年份，确保Year字段始终是目标年份，不会丢失
• 对每个年份，过滤nameList中对应年份的记录，提取SchoolNumber后去重，最后统计数量，就是该年份的不同院校数
• 最后按NumberOfSchools排序，和原逻辑一致

这样处理后，即使某个年份没有对应的毕业生数据，Year也会正确保留yearList中的值，NumberOfSchools为0，完全符合预期。

内容的提问来源于stack exchange，提问作者Fatality99